I know that feeling. You get so hopeful, you're happy they finally seem to "get it", then the whole bottom falls out your world.
Does he say he will take them? or is he outright refusing?
I feel like our positions are reversed - just a few months ago, your son was doing well on the shots & mine was either outright refusing the meds or lying about taking them. It's a good remind of how quickly things can change either way.
At least if he does end up back in the hospital, you know they'll most likely put him back on the shots & that would be good. You were asking about whether or not you could give them yourself, and not that I've seen them, I bet you could. We had to give our cat intramuscular shots before - the only problem was she was skin and bones at the time.
I also took some shots like that at home before - a long time ago. I gave some to myself in my thigh with an insulin needle. The ones that had to go deeper, a family member gave me - it wasn't hard at all.
I'm not going that route (my son already said he could probably give himself one) because I want him as connected as possible to this treatment program.